Smectite clay is a commercially important mineral. With the appropriate processing, smectite clays are excellent viscosifiers, binders, film formers, fabric softeners and retention aid additives in paper making. These clays are platey-type materials having a micaceous structure. They are highly colloidal and readily swell in water to form viscous, thixotropic gels which renders these clays useful as viscosity builders in many industries and applications including, for example, in foundry molding compositions, as suspension aids in agricultural sprays, and for thickening cosmetics. The high surface area generated by swelling in water also makes these clays useful in fabric softening and paper making applications.
Most smectite clays are sold as fine powders. As with most minerals, however, these powders are difficult and expensive to handle. They also are subject to considerable "dusting," i.e. evolution of dust during handling, so that they can, in some cases, cause environmental problems, and even health risks. To minimize these problems, many minerals are sold by the manufacturer as high solids aqueous slurries. Such high solids slurries can be easily stored, shipped, transferred, e.g. pumped and metered, with significantly less capital expenditure and many fewer problems than are associated with mineral dusts, as mentioned above.
In most applications, nevertheless, it is not economical to ship smectite clay slurries because of the large quantity of water present in shippable slurries. Since smectites are indeed good viscosifiers, it is not possible to produce high solid slurries by the usual methods. In general, only about 8% -10% solids slurries of good quality swelling smectite can be produced in water. Indeed, at solids contents greater than about 8%, the viscosities of the slurries can become so high that they cannot readily be pumped by conventional equipment and gelling upon standing becomes a problem. At higher solids it becomes virtually impossible to form a uniform paste without special equipment.
Thus, there is a need for slurries containing substantially greater than 8% by weight of clay, which have viscosities low enough to allow pumping. In addition, the more clay which can be incorporated into the slurry the more economical it is to ship the clay since the total weight of the slurry will include less water weight.
Van Fisk, U.S. Pat. No. 4,359,339 proposes the addition of water soluble aluminum salt, such as aluminum sulfate, to permit the making of pumpable aqueous slurries of bentonite of up to 26% by weight of clay, primarily for use in making foundry sand compositions used in the making of molds. While apparently suitable for the purpose disclosed, aluminum salts have not been found to be satisfactory for making clay slurries of higher solids content, which are frequently desired for many purposes, especially when shipping is involved. Van Fisk also points out that certain other ions, such as calcium ion have been known to depress the gelling properties of bentonites in aqueous slurries, but that the properties are not readily reversible upon later dilution, thus limiting the value of such use. While aluminum ion appears to be less subject to this objection, to a considerable extent it suffers from the same problem. All multivalent cations tend to strongly bind the clay platelets together, which prevents dispersion and inhibits performance in applications where good dispersion is needed (viscosifiers, fabric softeners, retention aids).
In accordance with the foregoing, it may be regarded as an object of the present invention, to provide a method for preparing a time-stable low viscosity, high solids aqueous bentonite slurry.
It is a further object of the invention to provide a method as aforementioned, wherein the bentonite in the resultant slurry, upon dilution in the fresh water at the point of use, can be easily dispersed to generate the large surface area needed for maximum performance in many applications.
It is a further object of the present invention to provide an improved smectite clay slurry of increased solids content.
It is a still further object of the invention to provide a smectite clay slurry of the character indicated which remains pumpable and shippable without prohibitive gelling even at elevated solids content.